You can buy them from the market board if you have the money. You'll need a Longarm's Composite Bow and two Heaven's Eye 3 Materia. You could have the NPC meld the materia for you.

If you can't afford it you can level up Carpenter class to 50 to make the bow. You can also make the materia yourself by desynthing spirit bonded weapons. The materia you get seems random though so that can take a long time.

I made the bow but I bought the materia. Good luck whatever you decide.